1. Field of the Invention
The present invention relates to a photodiode array for near infrared rays that includes photodiodes that can be easily and appropriately spaced apart during manufacture thereof and that has high selectivity for the wavelength of received light, a method for manufacturing the photodiode array, and an optical measurement system.
2. Description of the Related Art
Existing photodiode arrays are manufactured by forming semiconductor layers including an absorption layer on a semiconductor substrate, for example, by epitaxial growth and then separating photodiodes constituting the photodiode array by etching the semiconductor layers between the photodiodes.
For example, the semiconductor layers between the photodiodes are etched by wet etching using an etchant (see, for example, Japanese Unexamined Patent Application Publication No. 2001-144278). Furthermore, it is also known that trench grooves are formed by inductively coupled plasma etching, which is one type of dry etching, for device isolation (see, for example, Japanese Unexamined Patent Application Publication No. 2001-352094).
Furthermore, a group III-V element containing nitrogen (N) is used as an absorption layer that can receive near infrared rays having a wavelength in the range of 1.7 μm. to 5 μm as the longest wavelength to be detected (see, for example, Japanese Unexamined Patent Application Publication No. 9-219563).
However, the device isolation by wet etching has the following problems: (1) etching proceeds not only in the depth direction, but also in the transverse direction of the semiconductor layers, making it difficult to form photodiodes having the same size in the photodiode array, and (2) for example, in a laminating structure of semiconductors composed of different materials, such as InP/InGaAs/InP, a difference in etching rate between InP and InGaAs results in a warped semiconductor layer after etching.
On the other hand, device isolation by dry etching may cause damage, which prevents dark current from being reduced.
In these photodiode arrays, when high selectivity for the wavelength of received light is desired between the photodiodes, a wavelength filter may be placed in front of the photodiodes to select the wavelength. However, an apparatus including such a photodiode array may become larger.
Furthermore, when GaInNAs is used as the material of the absorption layer for near infrared rays, it is believed that an increase in nitrogen content increases a band gap wavelength, thus providing a photodiode sensitive to a wavelength of about 5 μm. However, a semiconducting crystal is hardly doped with nitrogen in a thermal equilibrium state. It is therefore difficult to prepare a semiconducting crystal containing nitrogen as much as several percent. Hence, it is not easy to prepare a photodiode sensitive to a wavelength of about 5 μm.